Light emitting device

ABSTRACT

It is an object of the invention to provide a light emitting device which can display a superior image in which luminescent color from each light emitting layer is beautifully displayed and power consumption is lowered in a light emitting element in which light emitting layers are stacked. One feature of the invention is that, in a light emitting element which comprises light emitting layers stacked between electrodes, each distance between each light emitting layer and an electrode is approximately oddly multiplied ¼ wavelength by controlling a thickness of a layer provided therebetween to enhance luminous output efficiency. Another feature of the invention is that a drive voltage is lowered using a high conductive material for the layer compared with a conventional element.

This application is a continuation of application Ser. No. 10/590,041filed on Aug. 18, 2006 now abandoned which is the US national stage ofPCT/JP2006/300921 filed Jan. 17, 2006.

TECHNICAL FIELD

The present invention relates to a light emitting device which can beutilized as a display means, a light source, or the like.

BACKGROUND ART

A display device including a light emitting element (hereinafter,referred to as a light emitting device) has a wider viewing angle and ahigher response characteristic and operates with lower power consumptioncompared with a display device having a liquid crystal element, namely aliquid crystal display device. Therefore, the light emitting device hasbeen actively developed.

A light emitting element includes an organic material or an inorganicmaterial between a pair of electrodes. By applying current to theorganic material or inorganic material and exciting a light emittingmaterial, a predetermined luminescent color can be obtained. To increaseemission luminance of the light emitting element, a large amount ofcurrent may be supplied, in other words, high voltage may be applied tothe pair of electrodes; therefore, the advantage of low powerconsumption cannot be attained. In addition, deterioration of the lightemitting element may be accelerated by applying a large amount ofcurrent.

Hence, a light emitting element in which emission luminance is increasedby stacking a plurality of light emitting elements and applying currentwhich has the same current density as a single layer is proposed (referto Patent Document 1: Japanese Patent Laid-Open No. 2003-272860). Byutilizing this light emitting element, a predetermined luminance can beobtained by a light emitting element having a stacked-layer structureeven if current which has less than half current density of a singlelayer is applied. For example, it is said that n times luminance can beaccomplished without increasing current density if n light emittingunits having the same structure existing between electrodes areprovided. At this time, it is mentioned that drive voltage also becomesn times or more; however, there is a great advantage that n timesluminance can be obtained without sacrificing lifetime.

DISCLOSURE OF INVENTION

The above Patent Document 1 discloses all optical film thicknesses fromeach emission position to a reflective electrode are set to beapproximately oddly multiplied ¼ wavelength since a plurality ofemission positions separately exists. Embodiment 5 in Patent Document 1discloses an optical distance from a blue emission position to areflective electrode is controlled intentionally by adjusting athickness of a hole transporting layer including α-NPD of a lightemitting unit in a light emitting element with a blue light emittingunit and a red light emitting unit.

The characteristics of a hole transporting layer such as α-NPD is closerto that of a light emitting layer as compared to a hole injecting layerand the hole transporting layer with low conductivity; therefore, astructure of increasing a thickness of the hole transporting layer isnot preferable since drive voltage is increased if the thickness of thehole transporting layer is increased.

The distance with which the luminous output efficiency not being loweredis different depending on an emission wavelength; therefore, the filmthickness of α-NPD in light emitting units are required to bedifferentiated from each other in alight emitting element. Therefore,luminous output efficiency of whole light emitting element is notenhanced only by differentiating film thicknesses of α-NPD in a bluelight emitting unit. Further, according to Patent Document 1, when filmthickness of α-NPD in a red light emitting unit is differentiated fromfilm thicknesses in the other units, a film thickness of whole lightemitting element is increased and a drive voltage is increased. Byincreasing drive voltage, a problem of increasing power consumption of alight emitting device is led.

Further, luminous efficiency is different according to each luminescentcolor. To take a balance of luminance in whole emission of a lightemitting device, it is required to apply a large amount of current for alight emitting element with inferior luminous efficiency; therefore,there is a disadvantage that deterioration of a light emitting elementis accelerated.

It is an object of the invention to reduce power consumption of a lightemitting device. It is another object of the invention to reducedeterioration because of luminance change of a pixel in a light emittingdevice. It is another object of the invention to provide a lightemitting device and a manufacturing method of the light emitting device,which can display a superior image in which beautiful luminescent colorfrom each light emitting layer is displayed and which operates with lowpower consumption in a light emitting element in which light emittinglayers are stacked.

In view of the foregoing problems, one feature of the present inventionis that, in a light emitting element which comprises light emittinglayers stacked between electrodes, an optical distance (hereinafter,referred to as a distance) from each light emitting layer to anelectrode is controlled. Specifically, one feature of the invention isthat the distance between a reflective electrode and each light emittinglayer is controlled by a thickness of a layer provided therebetween,respectively, to enhance luminous output efficiency.

Another feature of the invention is to form a pixel portion having alight emitting element in which light emitting layers are stacked and alight emitting element in which a light emitting layer is single. Forexample, a light emitting element having a problem of low emissionluminance is formed by stacking light emitting layers, and another lightemitting element is formed with one light emitting layer.

A specific mode of the invention is a light emitting device in which aplurality of light emitting layers are stacked between a first electrodeand a second electrode which face each other, and a distance from eachof the plurality of light emitting layers to the first electrode isapproximately oddly multiplied ¼ wavelength (2m−1)λ/4 (m: naturalnumber) by controlling a thickness of a layer provided between eachlight emitting layer and the first electrode so that luminous outputefficiency is enhanced. Note that it may not be possible to be justoddly multiplied ¼ wavelength because of film formation accuracy in somecases, therefore, “approximately” is used here. The invention comprisesthe range of oddly multiplied ¼ wavelength ±10% when recitingapproximately oddly multiplied ¼ wavelength.

Another mode of the invention is a light emitting device in which aplurality of light emitting layers are stacked between a first electrodeand a second electrode which face each other, each light emitted fromthe plurality of light emitting layers has different color, and adistance from each of the plurality of light emitting layers to thefirst electrode is approximately oddly multiplied ¼ wavelength bycontrolling a thickness of a layer provided between the light emittinglayer and the first electrode and in contact with each of the pluralityof light emitting layers so that luminous output efficiency is enhanced.

Another mode of the invention is a light emitting device including astacked layer type light emitting element in which a plurality of lightemitting layers is stacked between a first electrode and a secondelectrode which face each other and a single-layer type light emittingelement having one light emitting layer between a first electrode and asecond electrode, in which al distance from each of the plurality oflight emitting layers to the first electrode is approximately oddlymultiplied ¼ wavelength by controlling a thickness of a layer providedbetween the light emitting layer and the first electrode in the stackedlayer type light emitting element so that luminous output efficiency isenhanced.

Another mode of the invention is a light emitting device including astacked layer type light emitting element in which a plurality of lightemitting layers are stacked between a first electrode and a secondelectrode which face each other and a single-layer type light emittingelement having one light emitting layer between a first electrode and asecond electrode, in which each light emitted from the plurality oflight emitting layers in the stacked layer type light emitting elementhas different color, and the distance from each of the plurality oflight emitting layers to the first electrode is approximately oddlymultiplied ¼ wavelength by controlling a thickness of a layer which isprovided between the light emitting layer and the first electrode andwhich is in contact with each of the plurality of light emitting layersin light emitting element so that luminous output efficiency isenhanced.

One feature of the invention is that a highly conductive material isused for a layer for control so that luminous output efficiency isenhanced and the distance from a reflective electrode to a lightemitting layer is approximately oddly multiplied ¼ wavelength bycontrolling a thickness of the first layer. A film which determines adistance is formed from a highly conductive material in the lightemitting element according to the invention; therefore, drive voltagecan be lowered compared with the above Patent Document 1.

In the invention, a material which exhibits emission from a tripletexcited state or a material which exhibits emission from a singletexcited state can be included in a light emitting layer. Therefore,emission in which emission from a triplet excited state and emissionfrom a singlet excited state are included can be obtained by the stackedlight emitting layers. Obviously, only emission from a triplet excitedstate or only emission from a singlet excited state can be obtained fromthe stacked light emitting layers.

It is to be noted that a light emitting element including the stackedlight emitting layers can be referred to as a light emitting elementincluding n light emitting layers between electrodes by using a naturalnumber n.

According to the invention, luminance obtained when same amount ofcurrent is supplied can be increased compared with a light emittingelement having a light emitting layer of a single layer. In other words,the amount of current for obtaining same luminance, which flows betweenelectrodes, can be lowered.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a view showing a light emitting element according to thepresent invention;

FIG. 2 is a view showing a light emitting element according to theinvention;

FIG. 3 is a view showing a light emitting element according to theinvention;

FIG. 4 is a view showing a light emitting element according to theinvention;

FIG. 5 is a view showing a light emitting device according to theinvention;

FIG. 6 is a view showing a light emitting device according to theinvention;

FIG. 7 is a view showing a light emitting device according to theinvention;

FIGS. 8A to 8C are views showing light emitting devices according to theinvention;

FIGS. 9A to 9D are diagrams each showing a pixel circuit of a lightemitting device according to the invention;

FIG. 10 is a diagram showing a pixel circuit of a light emitting deviceaccording to the invention;

FIG. 11 is a view showing a television receiver according to theinvention;

FIG. 12 is a diagram showing a system of a television receiver accordingto the invention;

FIG. 13 is a diagram showing a television receiver according to theinvention;

FIGS. 14A to 14E are views each showing an electronic device accordingto the invention;

FIGS. 15A to 15C are views each showing a light emitting deviceaccording to the invention;

FIG. 16 is a graph showing luminance with respect to a wavelength oflight emitting elements;

FIG. 17 is a graph showing luminance with respect to a wavelength oflight emitting elements;

FIGS. 18A and 18B are views each showing a light emitting elementaccording to the present embodiment; and

FIG. 19 is a view showing a light emitting device according to theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment modes according to the present invention are described indetail with reference to the drawings. However, it is easily understoodby those who are skilled in the art that embodiments and details hereindisclosed can be modified in various ways without departing from thepurpose and the scope of the present invention. Therefore, it should benoted that the description of embodiment modes to be given below shouldnot be interpreted as limiting the present invention. Note that the samereference numerals are given to the same portions or the portions havingthe same function in all drawings, and the description thereof is notrepeated.

Embodiment Mode 1

In this embodiment mode, a structure of a light emitting element inwhich light emitting units are stacked is explained.

FIG. 1 shows a light emitting element in which a first light emittingunit 100B, a second light emitting unit 100G, and a third light emittingunit 100R are sequentially stacked between a first electrode 101 and asecond electrode 102. The colors of light emitted from the lightemitting units 100B, 100G, and 100R are not limited in particular. Inthis embodiment mode, however, a case where the first light emittingunit exhibits blue emission, the second light emitting unit exhibitsgreen emission, and the third light emitting unit exhibits red emissionis explained in this embodiment mode. The light emitting element inwhich light emitting units are stacked indicates a state where two ormore light emitting units are stacked. In this embodiment mode, a statewhere three light emitting units are stacked is explained; however, thepresent invention is not limited to this.

In the light emitting element shown in FIG. 1, the first electrode 101is formed from a material having high reflectivity and the secondelectrode 102 is formed from a material having a high light-transmittingproperty to extract light from the second electrode 102. The lightemitting unit 100R includes a first layer 111R, a second layer 112R, anda third layer 113R; the light emitting unit 100G includes a first layer111G, a second layer 112G, and a third layer 113G; and the lightemitting unit 100B includes a first layer 111B, a second layer 112B, anda third layer 113B.

Each of the light emitting units has a feature that a distance from thefirst electrode 101 to the second layer 112 (112B, 112G and 112R) ineach light emitting unit is approximately oddly multiplied ¼ wavelengthby controlling a thickness of the first layer 111 (111B, 111G and 111R)in the case of using each of the second layers 112 as a layer includinga light emitting layer. In other words, each of the light emittingelements has a feature that a distance between the first electrode 101and the light emitting layer is approximately oddly multiplied ¼wavelength by controlling a thickness of a layer provided therebetween.Therefore, the thicknesses of the first layers 111R, 111G, and 111B aredifferent in each light emitting unit.

The distances from the first electrode 101 to the respective lightemitting layers are different from one another since the light emittingunits are stacked so that luminous output efficiency is enhanced.Therefore, a light emitting element in which the thicknesses of thefirst layers 111 provided between the first electrode 101 and each lightemitting layer are controlled respectively is provided. As a result, astate where luminous output efficiency is high can be provided.

As described above, luminous efficiency can be enhanced by stackinglight emitting units; therefore, an amount of current flow can be keptlow to obtain same luminous and the lifetime of the light emittingelement can be improved.

In this embodiment mode, a mode in which the thicknesses of all thefirst layers 111R, 111G, and 111B are controlled is shown. However,according to the invention, a thickness of any one of the first layers111 included in a light emitting element in which light emitting unitsare stacked may be controlled. By controlling any one of the firstlayers 111, a state where luminous output efficiency is high can beprovided and an effect that an amount of current flow is kept low can beobtained.

In this invention, the light emitting element in which light emittingunits are stacked is not required to include light emitting layers eachof which exhibits different luminescent colors. In other words,according to the invention, layers which exhibit the same luminescentcolor may be stacked. This is because a state where luminous efficiencyis high can be provided even if layers which exhibit the sameluminescent colors are stacked, and an effect that an amount of currentflow is kept low can be obtained.

According to the invention, the first layer 111 is formed from a highlyconductive material and a thickness of the first layer 111 iscontrolled; therefore, drive voltage can be lowered compared with aconventional element mentioned in Patent Document 1 and the like.

The first to third layers 111 to 113 (113B, 113G and 113R) can be formedby a sputtering method, a vapor deposition method, or the like.

Next, the electrodes will be explained. The first electrode 101 isformed from a material having high reflectivity and the second electrode102 is formed from a material having a light-transmitting property. Thelight-transmitting property can be also obtained by forming a quite thinfilm using a material having no light-transmitting property.

As a material for the first electrode 101, a metal material such astitanium (Ti), aluminum (Al), gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), or palladium (Pd) can be used, and a single layer or astacked layer of the above metal material can be used. The firstelectrode 101 can be formed, for example, by a sputtering method, avapor deposition method, or the like.

The second electrode 102 can be formed from a light-transmittingmaterial such as indium tin oxide (ITO), indium tin oxide containingsilicon oxide, or indium oxide containing 2 to 20% of zinc oxide. Inaddition, it is possible to use a thin film formed from a metal materialhaving no light-transmitting property such as gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), or palladium (Pd) so as to have alight-transmitting property. As for the second electrode 102, a singlelayer or a stacked layer of the above metal materials can be used. Inthe case of using a stacked-layer structure, a structure in which themetal material having no light-transmitting property is thinly formedand a light-transmitting material is stacked thereover can be used. Inorder to prevent the resistance from being increased due to theformation of the thin second electrode 102, an auxiliary wiring can beprovided.

The first electrode 101 or second electrode 102 may be an anode or acathode, respectively, depending on voltage which is applied to thelight emitting element. It is preferable to use a material having a highwork function (work function of 4.0 eV or more) in case of an anode, anda material having a low work function (work function of 3.8 eV or less)in case of a cathode.

The first electrode 101 or second electrode 102 can be formed by asputtering method, a vapor deposition method, or the like. In the caseof using a vapor deposition method, the first electrode 101, the firstto third layers 111 to 113, and the second electrode 102 can be formedcontinuously without being exposed to the air. Impurity mixing intointerfaces and the like can be reduced by forming the light emittingelement continuously without being exposed to the air in this way.

The light emitting element according to the invention controls thethickness of the first layer 111 provided between each light emittinglayer and the first electrode 101, and thus, a state where luminousoutput efficiency is high can be obtained. Further, according to theinvention, luminous efficiency at the same current density can beenhanced by stacking the light emitting element; therefore, density ofcurrent flow can be kept low and the lifetime of the light emittingelement can be improved.

As described above, a light emitting device having a light emittingelement in which light emitting units are stacked can display an imagewhich is clear and superior in image quality, and low power consumptioncan be attained.

Embodiment Mode 2

In this embodiment mode, a structure of a light emitting element whichis different from that in the above-described embodiment mode isexplained.

According to the present invention, a light emitting element in whichlight emitting units are stacked is not required to be applied to alllight emitting elements formed over a substrate. A distance from each oflight emitting layers to a first electrode 101 can be approximatelyoddly multiplied ¼ wavelength by controlling a thickness of a firstlayer 111 for at least one light emitting element, and accordingly, astate where luminous output efficiency is high can be obtained, luminousefficiency at the same current density can be enhanced, and density ofcurrent which is applied can be kept low. As a result of keeping currentdensity low, the lifetime of the light emitting element can be improved.In this embodiment mode, a case where one light emitting element whichexhibits one luminescent color is a light emitting element in whichlight emitting units are stacked is explained.

FIG. 2 shows a state where a first light emitting element 100B, a secondlight emitting element 100G, and a third light emitting element 100R areprovided over the same substrate. The colors of light emitted from thelight emitting elements 100B, 100G, and 100R are not limited inparticular. In this embodiment mode, however, a case where the firstlight emitting element exhibits blue emission, the second light emittingelement exhibits green emission, and the third light emitting elementexhibits red emission, is explained.

A light emitting element in which light emitting units 100B (1) and 100B(2) are stacked is used for the light emitting element which exhibitsblue emission. A light emitting element in which light emitting unitsare stacked as described above is expediently referred to as a stackedlayer type light emitting element. Further, light emitting elements eachincluding one light emitting unit are used for the light emittingelements 100R and 100G which exhibit red emission or green emission,respectively, and are expediently referred to as single-layer type lightemitting elements.

The structures of the first electrode 101 and the second electrode 102are similar to that in the above-described Embodiment Mode 1; therefore,the explanation is omitted.

In such a stacked layer type light emitting element, the distance from alight emitting layer to the first electrode 101 is approximately oddlymultiplied ¼ wavelength by controlling a thickness of the first layer111 (111B(1) and 111B(2)). As a result, luminous output efficiency isenhanced and current density can be kept low. By keeping current densitylow, the lifetime of the stacked layer type light emitting element canbe improved. A second layer 112 (112B (1) and 112B(2)) and a third layer113 (113B(1) and 113B(2)) are included in each of the light emittingunits.

As described above, luminous efficiency can be enhanced by the stackedlayer type light emitting element; therefore, density of current flowcan be kept low and the lifetime can be improved. Therefore, it isdesirable to selectively apply the stacked layer type light emittingelement to a light emitting element which is easily deteriorated.

Also in a single-layer type light emitting element, the distance from alight emitting layer to a first electrode 101 can be approximately oddlymultiplied ¼ wavelength by controlling a thickness of a first layer 111(111G and 111R). As a result, luminous output efficiency can beenhanced. A second layer 112 (112G and 112R) and a third layer 113 (113Gand 113R) are included in each of the light emitting units.

In the stacked layer type light emitting element and single-layer typelight emitting element, the first layer 111 (111B(1), 111B(2), 111G and111R) is formed from a highly conductive material and a thickness of thefirst layer 111 is controlled; therefore, drive voltage can be loweredcompared with a conventional element mentioned in Patent Document 1 andthe like.

As described above, a light emitting device in which a stacked layertype light emitting element is used for at least one light emittingelement which exhibits one luminescent color can display an image whichis clear and superior in image quality, and low power consumption can beattained.

Embodiment Mode 3

In this embodiment mode, a case where a stacked layer type lightemitting element is applied to a light emitting element which exhibitsdifferent luminescent color from that in the above embodiment mode isexplained.

In the present invention, a stacked layer type light emitting elementmay be used for an element except a first light emitting element 100B.For example, a single layer type light emitting element may be used as alight emitting element 100G which exhibits luminescent color with highsensitivity with respect to human eyes such as green emission, andaccordingly, the light emitting element 100G may have a structure inwhich the number of light emitting layers included between a pair ofelectrodes is smaller than that of the stacked layer type light emittingelements 100R and 100B which exhibit red emission or blue emission,respectively.

The explanation of another structure is omitted since it is similar tothe above embodiment mode.

In the stacked layer type light emitting element, a distance from alight emitting layer to a first electrode 101 is approximately oddlymultiplied ¼ wavelength by controlling a thickness of a first layer 111.As a result, luminous output efficiency is enhanced and current densitycan be kept low. By keeping current density low, the lifetime of thestacked layer type light emitting element can be improved.

Also in a single-layer type light emitting element, a distance from alight emitting layer to a first electrode 101 can be approximately oddlymultiplied ¼ wavelength by controlling a thickness of a first layer 111.As a result, luminous output efficiency can be enhanced.

As described above, by differentiating the number of the light emittinglayers provided between the pair of electrodes in a light emittingelement which exhibits luminescent color with low sensitivity withrespect to human eyes and in a light emitting element which exhibitsluminescent color with high sensitivity with respect to human eyes,luminance of each color can be harmonized efficiently.

In the stacked layer type light emitting element and the single-layertype light emitting element, the first layer 111 is formed from a highlyconductive material and a thickness of the first layer 111 iscontrolled; therefore, drive voltage can be lowered compared with aconventional element mentioned in Patent Document 1 and the like.

As described above, a light emitting device in which a stacked layertype light emitting element is used for at least one light emittingelement which exhibits luminescent color can display an image which isclear and superior in image quality, and low power consumption can beattained.

Embodiment Mode 4

In this embodiment mode, a structure and a material of each lightemitting element are explained.

As shown in FIG. 3, a light emitting unit includes a first layer 111, asecond layer 112, and a third layer 113 which are stacked sequentiallyfrom a first electrode 101.

When voltage is applied to this light emitting element having the lightemitting unit so that the electric potential of the first electrode 101is higher than the electric potential of a second electrode 102, holesare injected from the first layer 111 into the second layer 112, andelectrons are injected from the third layer 113 into the second layer112. Then, holes and electrons are recombined in the second layer 112and a light emitting material is made to be an excited state, andaccordingly, luminescence is produced when the light emitting materialin the excited state returns to the ground state.

The material of the first to third layers 111 to 113 will be explained.

The first layer 111 is a layer which generates holes. This function canbe achieved by using a layer including a hole transporting material anda material which exhibits an electron accepting property to the holetransporting material. In addition, it is preferable that the materialwhich exhibits an electron accepting property to the hole transportingmaterial be included so that the molar ratio is 0.5 to 2 (=the materialwhich exhibits an electron accepting property to the hole transportingmaterial/the hole transporting material) with respect to the holetransporting material.

The hole transporting material is a material in which a transportingproperty of holes is higher than that of electrons, and for example,organic compounds such as aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′,4″-tris(N,N-diphenylamino) triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl(abbreviation: DNTPD); or phthalocyanine compounds such asphthalocyanine (abbreviation: H₂Pc), copper phthalocyanine(abbreviation: CuPc), and vanadyl phthalocyanine (abbreviation: VOPc)can be used. It is to be noted that the hole transporting material isnot limited thereto.

In addition, for example, an oxide of metal belonging to any one ofGroup 4 to 12 in the periodic table (a metal oxide) can be used as thematerial which exhibits an electron accepting property to the holetransporting material. Among others, an oxide of metal belonging to anyone of Groups 4 to 8 in the periodic table often has a high electronaccepting property, and a vanadium oxide, a molybdenum oxide, a niobiumoxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titaniumoxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and atantalum oxide are particularly preferable. Besides the oxides, nitridesand oxynitrides of the metals mentioned above may be used. It is to benoted that the material which exhibits an electron accepting property tothe hole transporting material is not limited thereto, and irontrichloride (FeCl₃), aluminum trichloride (AlCl₃), or7,7,8,8-tetracyano-2,3,5,6-tetrafluoro-quinodimethane (abbreviation:F₄TCNQ) may be used.

As described above, the first layer 111 includes a mixed layer of a holetransporting material comprising an organic compound and a materialwhich exhibits an electron accepting property to the hole transportingmaterial and which comprising the above metal oxide. It is to be notedthat the mixed layer includes a layer in which an organic compound andan inorganic compound are mixed or a layer in which each of an organiccompound and an inorganic compound are thinly formed.

By using this mixed layer of an organic compound and an inorganiccompound, crystallization of the organic compound can be suppressed andthe first layer 111 can be thickly formed without increasing resistance.Further, the mixed layer of an organic compound and a material whichexhibits an electron accepting property to the hole transportingmaterial and which is formed from the above metal oxide has highconductivity; therefore, a film can be thickened without increasingresistance. Hence, even if there is a depression/projection due to dust,dirt, or the like over the first electrode 101, thedepression/projection hardly impacts since the first layer 111 isthickly formed. Therefore, failure such as short circuit between thefirst electrode 101 and the second electrode 102 due to adepression/projection can be prevented. Further, the first electrode 101and the second layer 112 can be separated from each other by forming thefirst layer 111 thickly; therefore, quenching of emission due to metalcan be prevented.

It is to be noted that the first layer 111 may include another organiccompound. As another organic compound, rubrene and the like are given.Reliability can be enhanced by adding rubrene.

This first layer 111 can be formed by a vapor deposition method. When amixed layer of a plurality of compounds is formed as the first layer111, a co-evaporation method can be used. The co-evaporation methodincludes a co-evaporation method by resistance-heating evaporation, aco-evaporation method by electron-beam evaporation, and a co-evaporationmethod by resistance-heating evaporation and electron-beam evaporation.In addition, the first layer 111 may be formed by combining the sametype of methods or different types of methods, for example, depositionby resistance-heating evaporation and sputtering, and deposition byelectron-beam evaporation and sputtering. The example described aboveshows a layer including two kinds of materials. However, when three ormore kinds of materials are included, the first layer 111 may besimilarly formed by combining the same type of methods or differenttypes of methods.

Next, the second layer 112 which is a layer including a light emittinglayer is explained. The layer including the light emitting layer may bea single layer formed of only the light emitting layer or a multilayerincluding the light emitting layer. To cite a case, a specificmultilayer includes a light emitting layer and additionally an electrontransporting layer and/or a hole transporting layer. FIG. 3 shows a caseof a multilayer in which the second layer 112 includes a holetransporting layer 122, a light emitting layer 123, and an electrontransporting layer 124.

The hole transporting layer 122 can be formed from a known material.Typical examples include aromatic amine-based compounds, and forexample, star burst aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter, referredto as α-NPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine(hereinafter, referred to as TDATA), and4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(hereinafter, referred to as MTDATA) are given.

It is preferable that the light emitting layer 123 be a layer includinga light emitting material dispersed in a layer formed from a materialhaving larger energy gap than that of the light emitting material. It isto be noted that the energy gap indicates the energy gap between theLUMO level and the HOMO level. In addition, a material which provides afavorable luminous efficiency and is capable of producing luminescenceof a desired emission wavelength may be used for the light emittingmaterial.

For the material which is used for dispersing the light emittingmaterial, for example, anthracene derivatives such as9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA);carbazole derivatives such as 4,4′-bis(N-carbazolyl) biphenyl(abbreviation: CBP); metal complexes such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviation: Znpp₂), andbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: ZnBOX); and thelike can be used. However, the material which is used for dispersing thelight emitting material is not limited to these materials. By the lightemitting layer 123 in which the light emitting material is dispersed,quenching of emission from the light emitting material due toconcentration can be prevented.

Next, light emitting materials for the light emitting layer 123 will bementioned. When red emission is desired to be obtained,4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTB), periflanthene,2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene,and the like can be used. However, the material for obtaining redemission is not limited to these materials, and a material whichexhibits emission with a peak from 600 nm to 680 nm in an emissionspectrum can be used.

When green emission is desired to be obtained, N,N′-dimethylquinacridone(abbreviation: DMQd), coumarin 6, coumarin 545T, tris(8-quinolinolato)aluminum (abbreviation: Alq₃), and the like can be used. However, thematerial for obtaining green emission is not limited to these materials,and a material which exhibits emission with a peak from 500 nm to 550 nmin an emission spectrum can be used.

In addition, when blue emission is desired to be obtained,9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviation: DPA),9,10-bis(2-naphthyl) anthracene (abbreviation: DMA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato (abbreviation: BGaq),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), and the like can be used. However, the material for obtainingblue emission is not limited to these materials, and a material whichexhibits emission with a peak from 420 nm to 500 nm in an emissionspectrum can be used.

A light emitting device of full color display can be made by selectingsuch a light emitting material.

When white emission is desired to be obtained, for example, TPD(aromatic diamine),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), tris(8-quinolinolato) aluminum (abbreviation:Alq₃), Alq₃ doped with Nile Red which is red luminescent pigment, andAlq₃ are sequentially stacked from the first electrode 101 side by avapor deposition method or the like.

In addition, α-NPD, α-NPD doped with perylene,bis(2-methyl-8-quinolinolato)-4-phenylphenolate-aluminum (abbreviation:BAlq) doped with DCM1, and Alq₃ may be sequentially stacked from thefirst electrode 101 side by a vapor deposition method or the like.

In addition, white emission can be obtained by dispersing2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD) of 30 wt % as an electron transport agent intopoly(N-vinylcarbazole) (abbreviation: PVK) and dispersing an adequateamount of four kinds of pigments (TPB, coumarin 6, DCM1, and Nile Red).

Even if a light emitting device which displays single color of any oneof red, blue, green, and white is formed, desirable emission can beexhibited by a color filter, and further, full color display can beconducted.

As the light emitting layer 123, a layer in which a metal oxide is mixedinto an organic compound may be used. By using the mixed layer of anorganic compound and a metal oxide, the second layer 112 can be thicklyformed without increasing resistance.

Next, the electron transporting layer 124 is explained. The electrontransporting layer 124 is a layer which has a function of transportingelectrons injected from the second electrode 102 to the light emittinglayer 123. By providing the electron transporting layer 124 in this wayto further separate the second electrode 102 and the light emittinglayer 123 from each other, quenching of emission due to metal can beprevented.

It is preferable that the electron transporting layer 124 be formed froma material in which the electron mobility is higher than the holemobility. Further, it is more preferable that the electron transportinglayer 124 be formed from a material which has the electron mobility of10⁻⁶ cm²/Vs or more. In addition, the electron transporting layer 124may be a layer which has a multilayer structure formed by combining twoor more layers including the material described above. As a specificmaterial for the electron transporting layer, a metal complex having aquinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato) aluminum (abbreviation: Alq₃),tris(5-methyl-8-quinolinolato) aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviation: BeBq₂), orBAlq mentioned above, is preferred. In addition, a metal complex havingan oxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn (BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂),can be used. Moreover, besides metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-tri azole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproin (abbreviation: BCP), or the like can be also used.

This second layer 112 can be manufactured by a vapor deposition methodwhether the second layer 112 has a single-layer structure or astacked-layer structure. When a mixed layer is formed for layersincluded in the second layer 112, a co-evaporation method can be used.The co-evaporation method includes a co-evaporation method byresistance-heating evaporation, a co-evaporation method by electron-beamevaporation, and a co-evaporation method by resistance-heatingevaporation and electron-beam evaporation. In addition, the second layer112 can be formed by combining the same type of methods or differenttypes of methods, for example, deposition by resistance-heatingevaporation and sputtering and deposition by electron-beam evaporationand sputtering. The example described above shows a layer including twokinds of materials. However, when three or more kinds of materials areincluded, the second layer 112 can be formed also in the same way bycombining the same type of methods or different types of methods asdescribed above.

Next, the third layer 113 which is a layer generating electrons isexplained. As this third layer 113, for example, a layer including anelectron transporting material and a material which exhibits an electrondonating property to the electron transporting material can be cited.

It is to be noted that the electron transporting material is a materialin which a transporting property of electrons is higher than that ofholes, and for example, metal complexes such as tris(8-quinolinolato)aluminum (abbreviation: Alq₃), tris(4-methyl-8-quinolinolato) aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato) beryllium(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂), and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(abbreviation: Zn(BTZ)₂);2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7);3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ);3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ); bathophenanthroline (abbreviation: BPhen);bathocuproin (abbreviation: BCP); and 4,4′-bis(5-methyl-benzoxazol-2-yl)stilbene (abbreviation: BzOs) can be used. In addition, the third layer113 can be formed from an n-type semiconductor. However, the electrontransporting material is not limited thereto.

Further, as the material which exhibits an electron donating property tothe electron transporting material, a material selected from an alkalimetal or an alkaline earth metal, specifically, lithium (Li), calcium(Ca), natrium (Na), kalium (K), magnesium (Mg), or the like can be used.

Further, an oxide of the alkali metal, an oxide of the alkaline earthmetal, a nitride of the alkali metal, a nitride of the alkaline earthmetal, or the like can exhibit an electron donating property to theelectron transporting material. As a specific material, lithium oxide(Li₂O), calcium oxide (CaO), natrium oxide (Na₂O), kalium oxide (K₂O),magnesium oxide (MgO), and the like are given. As a material whichexhibits the similar effect, a nitride or a fluoride of the alkalimetal, and a nitride or a fluoride of the alkaline earth metal aregiven, and specifically, lithium fluoride (LiF), cesium fluoride (CsF),calcium fluoride (CaF₂), and the like can be given. However, thematerial which exhibits an electron donating property to the electrontransporting material is not limited thereto. It is preferable that thematerial which exhibits an electron donating property to the electrontransporting material be included so that the molar ratio is 0.5 to 2(=the material which exhibits an electron donating property to theelectron transporting material/the electron transporting material) withrespect to the electron transporting material.

Alternatively, the third layer 113 may be a layer formed from a materialsuch as zinc oxide, zinc sulfide, zinc selenide, tin oxide, or titaniumoxide.

Further, it is preferable that the third layer 113 include a mixed layerof the electron transporting material, which comprising an organiccompound, and the material which exhibits an electron donating propertyto the electron transporting material. Crystallization of the organiccompound which is used for the third layer 113 can be suppressed byusing this mixed layer of the organic compound and the inorganiccompound, and the third layer 113 can be thickly formed withoutincreasing resistance. Further, a mixed layer of an organic compound anda material which exhibits an electron donating property to the electrontransporting material and which is formed from the above metal oxide hashigh conductivity; therefore, a film can be thickened. Hence, even ifthere is a depression/projection due to dust, dirt, or the like over thesubstrate, the depression/projection hardly impacts since the thirdlayer 113 is thickly formed without increasing resistance. Therefore,failure such as short circuit between the first electrode 101 and thesecond electrode 102 due to a depression/projection can be prevented.Further, the first electrode 101 and the second layer 112 can beseparated from each other by forming the third layer 113 thickly;therefore, quenching of emission due to metal can be prevented.

Further, as the material which exhibits an electron donating property tothe electron transporting material, a material selected from an alkalimetal or an alkaline earth metal, specifically, an oxide of metalselected from lithium (Li), calcium (Ca), natrium (Na), kalium (K),magnesium (Mg), or the like is given. As a specific metal oxide, anoxide of the alkali metal or an oxide of the alkaline earth metal isgiven. Specifically, lithium oxide (Li₂O), calcium oxide (CaO), natriumoxide (Na₂O), kalium oxide (K₂O), magnesium oxide (MgO), and the likeare given. As a material which exhibits the similar effect, a nitride ora fluoride of the alkali metal, and a nitride or a fluoride of thealkaline earth metal are given, and specifically, lithium fluoride(LiF), cesium fluoride (CsF), calcium fluoride (CaF₂), and the like aregiven. In addition, an oxynitride of the metal may be used if thesimilar effect can be obtained.

This third layer 113 can be manufactured by a vapor deposition method.When a mixed layer is formed as the third layer 113, a co-evaporationmethod can be used. The co-evaporation method includes a co-evaporationmethod by resistance-heating evaporation, a co-evaporation method byelectron-beam evaporation, and a co-evaporation method byresistance-heating evaporation and electron-beam evaporation. Inaddition, the third layer 113 can be formed by combining the same typeof methods or different types of methods, for example, deposition byresistance-heating evaporation and sputtering and deposition byelectron-beam evaporation and sputtering. The example described aboveshows a layer including two kinds of materials. However, when three ormore kinds of materials are included, the third layer 113 can be formedalso in the same way by combining the same type of methods or differenttypes of methods as described above.

In the stacked layer type light emitting element, the third layer 113provided between the light emitting layers also serves as a layer whichprevents excitation energy from moving toward only any one of the lightemitting layers. The third layer 113 is preferably a layer which has ahigher ionization potential than that of a light emitting layer formedbelow the third layer 113 and which has a higher LUMO level than theLUMO level of a light emitting layer formed above the third layer 113.

Thus, the third layer 113 is preferably formed so as to have a filmthickness of 1 to 30 nm.

In the stacked layer type light emitting element, each of the firstlayers 111 and the third layers 113 may comprise a bipolar material. Abipolar material is a substance in which a value of a ratio of mobilityof one carrier which is any of an electron and a hole to mobility of theother carrier is 100 or less, preferably 10 or less, when mobility ofone carrier and mobility of the other carrier are compared with eachother. In particular, among bipolar materials, a material in whichmobility of a hole and an electron is 1×10⁻⁶ cm²/Vs or more ispreferably used. As the bipolar material, for example,2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviation: TPAQn),2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f, h]quinoxaline(abbreviation: NPADiBzQn), and the like are given. Further, each of thefirst layers 111 and the third layers 113 may comprises the same bipolarmaterial.

It is to be noted that one feature of the present invention is thatthicknesses of the first layers 111 in each light emitting element areapproximately oddly multiplied ¼ wavelength so that luminous outputefficiency is high, and the invention is not limited to the structure ofthe light emitting element shown in FIG. 3. For example, although FIG. 3shows the structure provided with the electron transporting layer 124formed in contact with the third layer 113, there may be a case wherethe electron transporting layer 124 is not included. In this case, thelight emitting layer 123 is in contact with the third layer 113;therefore, a material for dispersing a light emitting material ispreferably used for the light emitting layer 123. Similarly, there maybe a case where the hole transporting layer 122 is not included.

In addition, a material which is capable of producing luminescencewithout being dispersed, such as Alq₃, can be used for the lightemitting layer 123. Since Alq₃ or the like is a light emitting materialwhich has a favorable carrier transporting property, a layer composed ofonly Alq₃ can serve as the light emitting layer 123 without beingdispersed.

These first to third layers 111 to 113 can be formed by the same methodsuch as a vapor deposition method, and can be therefore formedcontinuously without being exposed to the air. Impurity mixing into aninterface and the like can be reduced by forming the first to thirdlayers 111 to 113 continuously without being exposed to the air in thisway.

FIG. 4 shows a structure which is different from that in FIG. 3.

As shown in FIG. 4, a light emitting element shown in this embodimentmode includes a first layer 111, a second layer 112, a third layer 113,and a fourth layer 128 which are sequentially stacked from a firstelectrode 101, and the structure has a feature of providing the fourthlayer 128. The fourth layer 128 can be formed from a material which isthe same as that of the first layer 111. Since another structure isdescribed above, the explanation is omitted.

By thusly providing the fourth layer 128, damage to each layer when asecond electrode 102 is formed can be reduced.

In a light emitting element in which light emitting units are stacked,the thicknesses of the first layers 111 in each light emitting elementare differentiated so that luminous output efficiency is enhanced. Inthe case of differentiate thicknesses, a mixed layer of an organiccompound and a metal oxide may be used for the fourth layer 128 in thesame manner as the first layer 111. It is preferable that the mixedlayer be used for the fourth layer 128 since, by using the mixed layerfor the fourth layer 128, drive voltage does not rise even if a film isthickened. It is to be noted that a nitride or an oxynitride of metalmay be used if an effect which is equivalent to a metal oxide can beobtained.

In addition, damage caused when the second electrode 102 is formed canbe expected to be further reduced by thickening the fourth layer 128.

The invention can provide a light emitting device in which the mixedlayer of an organic compound and a metal oxide is used for the firstlayer 111 and the fourth layer 128 in the light emitting element inwhich light emitting units are stacked, luminous output efficiency isenhanced by a thickness of the layer, and driving at low voltage can beachieved. Further, the light emitting layer 123 and the first electrode101, or the light emitting layer 123 and the second electrode 102 can beseparated from each other by forming the layers thickly; therefore,quenching of emission due to metal can be prevented. Furthermore, thelight emitting element can be thickly formed; therefore, short circuitbetween electrodes can be prevented and mass productivity can beenhanced.

A light emitting device having a stacked layer type light emittingelement having the above described layer can display an image which isclear and superior in image quality, and low power consumption can beattained.

Embodiment Mode 5

In this embodiment mode, a cross-sectional view of three pixels isexplained with reference to FIG. 5, where a transistor (drivingtransistor) which controls current supply toward a light emittingelement is a p-type thin film transistor (TFT) 611 and light emittedfrom a light emitting element 603 is extracted from a second electrode102 side (top emission type). In this embodiment mode, light emittingunits 100R, 100G, and 100B which exhibit emission of R, G, and B,respectively, are stacked; therefore, colors are mixed and light emittedfrom the light emitting element can be recognized as white light.Therefore, a mode that full color display is conducted by using colorfilters 612R, 612G, and 612B of each color, which are formed over anopposing substrate 610, is shown.

Besides, a light emitting element in which light emitting units arestacked which exhibits white color has a structure in which a lightemitting unit which exhibits red emission and a light emitting unitwhich exhibits bluish green emission are stacked. By mixing the colors,light emitted from the light emitting element can be recognized as whitelight.

In FIG. 5, a p-type TFT 611 is formed over a substrate 600 and a firstelectrode 101 and the TFT 611 are electrically connected. Further, lightemitting units which exhibit emission of each of R, G, and B are stackedand a second electrode 102 is stacked over the first electrode 101.There is a case where the structure of a light emitting element includesfirst to third layers 111 to 113 as shown in the above embodiment modeand further includes a fourth layer 128.

The TFT 611 has a source region and a drain region added with animpurity element and a channel forming region formed in a separatedisland-shaped semiconductor film which is 10 to 200 nm in thickness. Forthe semiconductor film, any of an amorphous semiconductor film, acrystalline semiconductor film, and a microcrystalline semiconductorfilm may be used. For example, in case of a crystalline semiconductorfilm, a crystalline semiconductor film obtained by forming an amorphoussemiconductor film first and then conducting heat treatment can be used.The heat treatment indicates treatment using a heating furnace, laserirradiation, irradiation with light emitted from a lamp (hereinafter,referred to as lamp annealing) instead of laser light, or a combinationthereof. In the case of using laser irradiation, a continuous-wave laser(CW laser) or a pulsed-oscillation laser (pulsed laser) can be used, andfurther, these lasers can be used by being combined. For example, alaser light of a continuous-wave fundamental wave and a laser light of acontinuous-wave harmonic may be emitted, and alternatively, a laserlight of a continuous-wave fundamental wave and a laser light of apulsed-oscillation harmonic may be emitted. By emitting a plurality oflaser lights, energy can be compensated.

Further, in case of laser irradiation, an incidence angle of laser maybe set to be θ (0°<θ<90°) with respect to the semiconductor film. As aresult, laser interferometry can be prevented.

Alternatively, a pulsed laser may be used, which can obtain acontinuously-grown crystal grain in a scanning direction by oscillatinga laser light at a repetition rate which can emit the next pulsed laserlight during a period from melting the semiconductor film by laser lightto solidifying the same. A pulsed beam which is emitted at a frequencywith a lower limit can be used in such a way that the period of pulsedoscillation is shorter than the period from melting the semiconductorfilm to completing solidification thereof. The repetition rate of apulsed beam which can be actually used is 10 MHz or more, and afrequency band much higher than a usually-used frequency band of severaltens to several hundreds of Hz is used.

As another crystallization method by heat treatment, in the case ofusing a heating furnace, there is a method of heating an amorphoussemiconductor film at 500 to 550° C. for 2 to 20 hours. In this case,the temperature is preferably controlled by multistep regulation in the500 to 550° C. range so as to gradually get higher. Since hydrogen andthe like in the semiconductor film are released in the initial heatingstep at a lower temperature, film roughness by crystallization can bereduced, and further, a dangling bond can be terminated. Moreover, it ispreferable to provide a metal element which promotes crystallization,for example, Ni, on the amorphous semiconductor film since the heatingtemperature can be reduced. Even in case of crystallization using thismetal element; the semiconductor film may be heated to 600 to 950° C.

However, in the case of forming the metal element, there is fear thatadverse effects are caused on electrical characteristics of asemiconductor element. Therefore, it is necessary to perform a getteringstep for reducing or removing the metal element. For example, a step ofcapturing the metal element with an amorphous semiconductor film as agettering sink may be conducted.

Further, the TFT 611 has a gate insulating film covering thesemiconductor film and a gate electrode, and an insulating filmcontaining hydrogen may be provided over the gate electrode. A danglingbond in a crystalline semiconductor film can be terminated by thehydrogen.

The TFT 611 has a single-drain structure including onlyhigher-concentration impurity regions, the source region and the drainregion. Alternatively, the TFT 611 may have an LDD (Lightly Doped Drain)structure including a lower-concentration impurity region andhigher-concentration impurity regions. It is to be noted that the TFT611 may have a GOLD (Gate Overlapped LDD) structure in which alower-concentration impurity region is overlapped with a gate electrode.

The TFT 611 is covered with an interlayer insulating film 607, and aninsulating film 608 with an opening is formed over the interlayerinsulating film 607. In this embodiment mode, the invention is notlimited to the structure having the interlayer insulating film 607 andthe insulating film 608, and a structure having only the interlayerinsulating film 607 may be employed. The interlayer insulating film 607may be a single-layer structure or a stacked-layer structure, and can beformed from an inorganic material, an organic material, or astacked-layer structure of an inorganic material and an organicmaterial. When an organic material is used, planarity can be enhanced.As the organic material, polyimide, acrylic, polyamide, polyimide amide,resist, or benzocyclobutene can be used. Further, siloxane orpolysilazane may be used for the interlayer insulating film 607.Siloxane is an insulating film including a Si—O—Si bond formed by usinga siloxane-based material as a starting material. Polysilazane is aninsulating film formed by using a liquid material containing a polymermaterial having a bond of silicon (Si) and nitrogen (N) as a startingmaterial. In this embodiment mode, a structure in which the interlayerinsulating film 607 is formed from an inorganic material is shown.

The first electrode 101 is partly exposed in an opening portion of theinsulating film 608, and a stacked layer type light emitting element inwhich the first electrode 101, the light emitting units 100B, 100G, and100R, and the second electrode 102 are sequentially stacked in theopening is formed.

In each of the light emitting units 100B, 100G, and 100R, a thickness ofeach first layer 111 is approximately oddly multiplied ¼ wavelength sothat luminous output efficiency from each light emitting layer 123 isenhanced. In addition, a layer in which an organic compound and a metaloxide are mixed is used for the first layer 111 to prevent drive voltagefrom rising due to increase in thickness.

As described above, each light emitting unit includes the first layer111 having a hole transporting material, the third layer 113 having anelectron transporting material, and the like, in addition to the secondlayer 112 having the light emitting layer 123.

In this embodiment mode, the first electrode 101 is an anode and thefirst layer 111 having a hole transporting material, the second layer112 having the light emitting layer 123, and the third layer 113 havingan electron transporting material are sequentially stacked from thefirst electrode 101 since the TFT 611 is a p-channel type.Alternatively, when the TFT 611 is an n-type, the first electrode 101 ispreferably a cathode, and the third layer 113 having an electrontransporting material, the second layer 112 having the light emittinglayer 123, and the first layer 111 having a hole transporting materialare sequentially stacked from the first electrode 101.

In this embodiment mode, a top emission type is employed; therefore, thefirst electrode 101 has reflectivity (namely, non light-transmittingproperty) and the second electrode 102 is formed from alight-transmitting material. For these materials, it is possible torefer to the embodiment mode described above.

In case of the pixel shown in FIG. 5, light emitted from the lightemitting element 603 can be extracted from the second electrode 102 sideas indicated by an arrow, and full color display can be conducted by thecolor filters 612R, 612G, and 612B.

According to the invention, white emission with a wide range in whichemission wavelengths of respective R, G, and B are added with oneanother can be obtained compared with the light emitting element formedfrom a white color material having a single layer. Further, in a lightemitting unit of each of R, G, and B, a thickness of the first layer 111is approximately oddly multiplied ¼ wavelength so that luminous outputefficiency is enhanced. Therefore, thicknesses of the first layers 111are different from one another in accordance with each of R, G, and B,and a layer in which an organic compound and a metal oxide are mixed ispreferably used for the first layer 111 which is required to beespecially thickened. This is because drive voltage can be preventedfrom rising even if the thickness of the first layer is increased.

In this embodiment mode, the TFT 611 can be made to be an n-type. Inthis case, the third layer 113 having a hole transporting material, thesecond layer 112 having the light emitting layer 123, and the firstlayer 111 having a hole transporting material may be sequentiallystacked from the first electrode 101 by making the first electrode 101serve as a cathode.

Embodiment Mode 6

In this embodiment mode, a mode of conducting full color display bycolor filters 612R, 612G, and 612B of each color which are formed belowa substrate 600 is shown, where a transistor 611 is a p-type and lightemitted from a light emitting element 603 is extracted from a firstelectrode 101 side (bottom emission type).

In FIG. 6, a first electrode 101 of a light emitting element 603 and aTFT 611 are electrically connected. In addition, light emitting units100B, 100G, and 100R and a second electrode 102 are stacked over thefirst electrode 101.

The TFT 611 can be formed in the same manner as in the above embodimentmode. Also in this embodiment mode, the invention is not limited to astructure having an interlayer insulating film 607 and an insulatingfilm 608, and a structure having only an interlayer insulating film 607may be employed. Further, since a bottom emission type is employed, thefirst electrode 101 has a light-transmitting property and the secondelectrode 102 has reflectivity. For these materials, it is possible torefer to the embodiment mode described above. Furthermore, color filters612R, 612G, and 612B of each color are provided below the substrate 600in the first electrode 101 side which is in a light emission side. It isto be noted that the color filters 612R, 612G, and 612B are not requiredto be provided below the substrate 600 and may be provided in a lightemitting direction. For example, the color filters 612R, 612G, and 612Bcan be provided in the same layer as in the interlayer insulating film607.

The light emitting element 603 in which light emitting units are stackedcan be formed in the same manner as in the above embodiment mode. Inother words, in each of the light emitting units 100B, 100G, and 100R, athickness of each of the third layer 113 is approximately oddlymultiplied ¼ wavelength so that luminous output efficiency from eachlight emitting layer 123 is enhanced. Further, a layer in which anorganic compound and a metal oxide are mixed is used for a third layer113 to prevent drive voltage from rising due to increase in thickness.As a specific material in the third layer 113, lithium oxide (Li₂O),calcium oxide (CaO), natrium oxide (Na₂O) kalium oxide (K₂O), magnesiumoxide (MgO), and the like are given. As a material which exhibits thesimilar effect, a nitride or a fluoride of the alkali metal, and anitride or a fluoride of the alkaline earth metal are given, andspecifically, lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), and the like can be given.

In case of the pixel shown in FIG. 6, light emitted from the lightemitting element 603 can be extracted from the first electrode 101 sideas indicated by an arrow, and full color display can be conducted by thecolor filters 612R, 612G, and 612B.

According to the invention, white emission with a wide range in whichemission wavelength of each of R, G, and B are added can be obtainedcompared with the light emitting element formed from a white colormaterial having a single layer. Further, in a light emitting unit ofeach of R, G, and B, a thickness of the third layer 113 is approximatelyoddly multiplied ¼ wavelength so that luminous output efficiency isenhanced. Therefore, thicknesses of the third layers 113 are differentfrom each other in accordance with each of R, G, and B, and a layer inwhich an organic compound and a metal oxide are mixed is preferably usedfor the third layer 113 which is required to be especially thickened.This is because drive voltage can be prevented from rising even if athickness is increased.

In the case where the TFT 611 is an n-type in this embodiment mode, thefirst electrode 101 is made to serve as a cathode. Therefore, the firstlayer having an electron transporting material, the second layer havingthe light emitting layer, and the third layer having a hole transportingmaterial may be sequentially stacked from the first electrode 101.

Embodiment Mode 7

In this embodiment mode, a case where full color display is conductedwithout using color filters by using light emitting materials whichexhibit each luminescent color for each of light emitting units 100R,100G, and 100B is explained.

A top emission type light emitting device shown in FIG. 7 is explained,in which a TFT 611 is a p-type and light emitted from a light emittingelement 603 is extracted from a second electrode 102 side. The TFT 611and each of light emitting elements 100R, 100G, and 100B are providedover a substrate 600. At this time, a stacked layer type light emittingelement 100B is used for blue (B), and single-layer type light emittingelements 100R and 100B are used for red (R) and green (G), respectively.Then, a thickness of a first layer 111 of each light emitting element isapproximately oddly multiplied ¼ wavelength to enhance luminous outputefficiency. In a stacked layer type light emitting element, a thicknessof the first layer 111 is approximately oddly multiplied ¼ wavelengthaccording to each light emitting element.

The blue light emitting element is stacked as described above since alight emitting element emitting blue color has low luminous efficiencycompared with light emitting elements emitting other colors, therefore,having short life time. Since luminous efficiency is low, it is requiredto drive at high voltage, and accordingly, deterioration is easy togenerate. By stacking the light emitting element, luminous efficiency atthe same current density can be enhanced; therefore, density of currentflow can be kept low and lifetime can be improved.

The structures of the TFT 611, an interlayer insulating film 607, and aninsulating film 608 are the same as that in the above embodiment mode;therefore, the explanation is omitted. In this case, a first electrode101 is formed from a material having reflectivity and the secondelectrode 102 is formed from a light-transmitting material.

Even if full color display is conducted as shown in FIG. 7, colorfilters 612R, 612G, and 612B may be provided over the substrate 600 oran opposing substrate 610 as shown in the above embodiment mode. Thewidth of emission spectrum can be narrowed and beautiful image can beprovided using the color filter.

It is to be noted that the TFT 611 may be an n-type, and in that case,the first electrode 101 is preferably a cathode. Then, a third layer 113having an electron transporting material, a second layer 112 having alight emitting layer 123, and the first layer 111 having a holetransporting material may be sequentially stacked from the firstelectrode 101.

The stacked layer type light emitting element may be used for a red (R)and green (G) light emitting elements other than blue (B). By stackingthe light emitting element, luminous efficiency at the same currentdensity can be enhanced; therefore, current density can be kept low andlifetime can be improved.

Embodiment Mode 8

In this embodiment mode, a mode of a module which can be connected to anexternal circuit in a light emitting device sealed by an opposingsubstrate 610 is explained. In this embodiment mode, not a structurehaving an interlayer insulating film 607 and an insulating film 608 buta structure having only an interlayer insulating film 607 is explained.The number of steps is reduced by employing the structure having onlythe interlayer insulating film 607; therefore, mass productivity can beenhanced.

FIG. 8A shows a light emitting device in which a pixel portion 720 overa substrate 600, a first scanning line driver circuit 721, a secondscanning line driver circuit 722, and a signal line driver circuit 723in the periphery of the pixel portion are integrated. The first scanningline driver circuit 721, the second scanning line driver circuit 722,and the signal line driver circuit 723 are connected to an externalcircuit respectively through a flexible printed circuit 716.

The light emitting device is sealed so that the light emitting elementis not exposed to the air directly. In this embodiment mode, sealing isconducted by bonding the substrate 600 and the opposing substrate 610 toeach other by a sealing material 712.

FIG. 8B shows a cross-sectional view of E-F in FIG. 8A. As shown in FIG.8B, an inside 751 which is sealed is hollow, and the inside may befilled with gas such as a nitrogen gas or an inert gas, or resin sinceintrusion of oxygen or moisture which causes deterioration of a lightemitting element can be prevented. Further, a signal input from theexternal circuit is input from a connection wire 705 to the firstscanning line driver circuit 721, the second scanning line drivercircuit 722, and the signal line driver circuit 723 through the flexibleprinted circuit 716, and an emitting state or non-emitting state can becontrolled according to each light emitting element by a signal from thedriver circuit to be displayed as an image. The flexible printed circuit716 and the connection wire 705 are connected to each other through ananisotropic conductive material 715.

Further, a spacer 752 is provided so as to keep a distance from thesubstrate 600 to the opposing substrate 610 which are bonded by thesealing material 712. Although the spacer 752 is provided over theinterlayer insulating film 607 of TFT 611, the invention is not limitedto this. In addition, the spacer 752 may be a columnar shape or aspherical shape.

In the case where a light emitting element 603 exhibits single coloremission or emission of R, G, and B, a color filter 612 may be providedover the opposing substrate 610. The position of the color filter 612may be a substrate 600 side, not an opposing substrate 610 side.Beautiful image can be provided using the color filter 612.

FIG. 8C also shows a cross-sectional view of E-F in FIG. 8A. FIG. 8C hasa different structure from FIG. 8B, in which the substrate 600 and theopposing substrate 610 are bonded not by the sealing material 712 but byresin 753. By bonding the substrates by the resin 753, inferiority informing the sealing material 712 can be eliminated. A drying agent maybe added to the resin 753. In addition, resin 753 having alight-transmitting property may be used according to an emissiondirection.

In this embodiment mode, the first scanning line driver circuit 721, thesecond scanning line driver circuit 722, and the signal line drivercircuit 723 may be mounted by a TAB (Tape Automated Bonding) method tothe substrate being formed only the pixel portion 720; the firstscanning line driver circuit 721, the second scanning line drivercircuit 722, and the signal line driver circuit 723 may be mounted byCOG (Chip On Glass) method to the pixel portion 720 and the peripherythereof; or the pixel portion 720, the first scanning line drivercircuit 721, and the second scanning line driver circuit 722 may beintegrated over the substrate 600, and the signal line driver circuit723 may be separately mounted as an IC. The light emitting deviceaccording to the invention can obtain the effect in any mode of a drivercircuit. Further, a TFT using a crystalline semiconductor film or a TFTusing an amorphous semiconductor film may be used in a driver circuit.For example, the first scanning line driver circuit 721 and the secondscanning line driver circuit 722 are not required to operate at highspeed compared with the signal line driver circuit 723; therefore, a TFTusing an amorphous semiconductor film can be used. A TFT using anamorphous semiconductor film can be also used for a part of thecircuits, for example, a buffer circuit, even in the signal line drivercircuit 723.

Embodiment Mode 9

In this embodiment mode, a mode of a light emitting device which isdifferent from the above embodiment mode is illustrated. In thisembodiment mode, not a structure having an interlayer insulating film607 and an insulating film 608 but having only an interlayer insulatingfilm 607 is explained. By employing the structure having only theinterlayer insulating film 607, the number of steps is reduced;therefore, mass productivity can be enhanced.

FIG. 15A shows a mode in which a top emission type light emitting deviceshown in FIG. 5 and a bottom emission type light emitting device shownin FIG. 6 are bonded to each other to be one light emitting device. Inthis case, a substrate 600 of the top emission type light emittingdevice can be used as an opposing substrate of the bottom emission typelight emitting device.

For example, the formation up to a light emitting element 603 isconducted in the bottom emission type light emitting device. Then, a topemission type light emitting device and the bottom emission type lightemitting device are bonded to each other by a sealing material 712. Atthis time, a top emission type light emitting device may be in a statewhere the opposing substrate 610 is pasted.

In the same manner as in the above embodiment mode, a space produced bythe bonding may be filled with gas such as a nitrogen gas or an inertgas, or resin. For example, an epoxy resin can be used as the resin. Anepoxy resin has adhesiveness; therefore, adhesive strength can beenhanced.

FIG. 15B shows a mode in which a top emission type light emitting deviceshown in FIG. 5 and a bottom emission type light emitting device shownin FIG. 6 are bonded to each other to be one light emitting device andin which a scanning line driver circuit or a signal line driver circuit723 is shared. Sharing a scanning line driver circuit or a signal linedriver circuit 723 means that wirings formation is conducted so that asignal is supplied from a scanning line driver circuit or a signal linedriver circuit 723 provided over the substrate 600 of one light emittingdevice to a light emitting element 603 provided over another lightemitting device. Therefore, another light emitting device can be bondedin a state where only a pixel portion 720 is provided over the substrate600.

In the same manner as in the above embodiment mode, a space produced bythe bonding may be filled with a nitrogen gas or an inert gas, or resin.For example, an epoxy resin can be used as the resin. An epoxy resin hasadhesiveness; therefore, adhesive strength can be enhanced.

In addition, a light emitting device which can be applied to theinvention is not limited to light emitting devices having differentemission directions such as a top emission type light emitting deviceand a bottom emission type light emitting device, and light emittingdevices having the same emission directions may be bonded to each other.For example, bottom emission type light emitting devices shown in FIG. 6can be bonded to each other to be one light emitting device. In thiscase, the emission direction is the same; therefore, bottom emissiontype light emitting devices are arranged so as to face each other, andbonded to each other by a sealing material 712 to emit light to outsidedirection. Similarly, top emission type light emitting devices shown inFIG. 5 can be bonded to each other to be one light emitting device. Inthis case, the substrates 600 may be bonded to each other to emit lightto outside direction.

In the case of bonding bottom emission type light emitting devices so asto face each other, the bonding can be conducted without providing theopposing substrate 610 as shown in FIG. 15C. As a result, thinning of alight emitting device can be achieved. For example, the formation up toa light emitting element 603 is conducted in two bottom emission typelight emitting devices. Then, the bottom emission type light emittingdevices are bonded to each other by a sealing material 712. Obviously,the opposing substrate 610 may be shared, and the bonding may beconducted in a state where the opposing substrate 610 is pasted in eachbottom emission type light emitting device.

In the same manner as in the above embodiment mode, a space produced bythe bonding may be filled with a nitrogen gas or an inert gas, or resin.For example, an epoxy resin can be used as the resin. An epoxy resin hasadhesiveness; therefore, adhesive strength can be enhanced.

This embodiment mode can be freely combined with the above embodimentmodes. For example, a structure in which sealing is conducted by resin753 without using the sealing material shown in FIG. 8C can be employedfor the light emitting device shown in this embodiment mode.

By using the light emitting device, a display device with high addedvalue can be attained and new application can be provided.

Embodiment Mode 10

In this embodiment mode, an equivalent circuit diagram of a pixelincluded in a light emitting device is explained with reference to FIGS.9A to 9D.

FIG. 9A shows an example of an equivalent circuit of a pixel, whichincludes a signal line 912, a power supply line 915, and a scanning line910, and a light emitting element 603, transistors 611 and 911, and acapacitor element 904. A TFT can be applied to the transistor.

In this equivalent circuit, a video signal is input from a signal linedriver circuit to the signal line 912. The transistor 911 is able tocontrol supply of the electric potential of the video signal to a gateof the transistor 611 in accordance with a selection signal which isinput to the scanning line 910, and is referred to as a switchingtransistor. The transistor 611 is able to control supply of current tothe light emitting element 603 in accordance with the electric potentialof the video signal, and is referred to as a driving transistor. Thelight emitting element goes into an emitting state or non-emitting statein accordance with supplied current, which makes it possible to displayimages. The capacitor element 904 is able to hold voltage between thegate and source of the transistor 611.

FIG. 9B is an equivalent circuit diagram of a pixel where a scanningline 919 and a transistor 918 are additionally provided to theequivalent circuit diagram of the pixel shown in FIG. 9A.

The transistor 918 makes it possible to make the electric potentials ofthe gate and source of the transistor 611 equal to each other so that astate where no current flows into the light emitting element 603 can beforcibly made, and is referred to as an erasing transistor. Therefore,in time gray-scale display, a video signal can be input before inputtingvideo signals into all pixels, and the duty ratio can be thus madehigher.

Alternatively, an element 938 which functions as a diode (diode element)may be provided instead of the erasing transistor 918 as shown in FIG.9C. Then, a state where no current flows into the light emitting element603 can be forcibly made in the same manner as a case of using theerasing transistor 918.

As an operation method, the scanning line 910 is selected to make thetransistor 911 an ON state, and a signal is input from the signal line912 to the capacitor element 904. Accordingly, current of the transistor611 is controlled in accordance with the signal and current flows fromthe power supply line 915 to the light emitting element 603 to emitlight. The voltage for making current flow to the light emitting element603 corresponds to a drive voltage.

In the case of erasing a signal, the scanning line 919 is selected tomake the diode element 938 be applied voltage so that gate voltage ofthe transistor 611 is made high. As a result, the driving transistor 611is made to be an OFF state. Accordingly, current does not flow from thepower supply line 915 to the light emitting element 603. Consequently,non-lighting period can be produced and the length of a lighting periodcan be freely controlled. Thus, the duty ratio can be made higher.

The diode element 938 is not limited and any elements with rectificationcan be used. The diode element may be a PN diode, a PIN diode, aSchottky diode, or a Zener diode, or a diode-junction (connection of agate electrode and an electrode on higher electric potential side)transistor may be used. In FIG. 9C, an N-type transistor ofdiode-junction (connection of a gate electrode and a drain electrode) isused as the diode element 938. However, the invention is not limited tothis, and a P-type transistor may be used. In the case of using theP-type transistor, the gate electrode and the source electrode areconnected to each other.

FIG. 9D is an equivalent circuit diagram of a pixel where a transistor925 and a wiring 926 are additionally provided to the equivalent circuitdiagram of the pixel shown in FIG. 9B. The gate of the transistor 925has a fixed electric potential by the wiring 926. In addition, thetransistors 611 and 925 are connected in series between the power supplyline 915 and the light emitting element 603. Therefore, in FIG. 9D, thetransistor 925 is able to control the value of current supplied to thelight emitting element 603 whereas the transistor 611 is able to controlwhether or not the current is supplied to the light emitting element603.

The equivalent circuits of the pixels shown in FIGS. 9A to 9D can bedriven by a digital method. In the case of driving by a digital method,some variations in electrical characteristics of each driving transistorare negligible, if any, since the transistors are used as switchingelements.

An equivalent circuit of a pixel of a light emitting device according tothe invention can be driven by either a digital method or an analogmethod. For example, an equivalent circuit of a pixel shown in FIG. 10includes a signal line 912, a power supply line 915, and a scanning line910, a light emitting element 603, transistors 911, 920, and 921, and acapacitor element 904. In FIG. 10, the transistors 920 and 921 which arep-type transistors form a current mirror circuit. In this equivalentcircuit of a pixel, in case of a digital method, a digital video signalis input from the signal line 912, and the value of current supplied tothe light emitting element 603 is controlled in accordance with a timegray-scale. Alternatively, in case of an analog method, an analog videosignal is input from the signal line 912, and the value of currentsupplied to the light emitting element 603 is controlled in accordancewith the value of the analog video signal. In the case of driving theequivalent circuit by the analog method, lower power consumption can beachieved.

In each pixel described above, signals are input to the signal line 912,the power supply lines 915 and wiring 926 from a signal line drivercircuit. In addition, signals are input to the scanning lines 910 and919 from a scanning line driver circuit. One or more signal line drivercircuits and one or more scanning line driver circuits can be provided.For example, first and second scanning line driver circuits can beprovided through a pixel portion.

In addition, in the pixel shown in FIG. 9A, a state where no currentflows into the light emitting element 603 can be forcibly made asdescribed with reference to FIG. 9B. For example, the transistor 911 isselected by a first scanning line driver circuit at the moment when thelight emitting element 603 lights up, and a signal for forcibly applyingno current into the light emitting element 603 is supplied to thescanning line 910 by a second scanning line driver circuit. The signalfor forcibly applying no current (Write Erase Signal) is a signal forapplying an electric potential so that first and second electrodes 101and 102 of the light emitting element 603 have the same electricpotential. In this way, a state where no current flows into the lightemitting element 603 can be forcibly made, and the duty ratio can bethus made higher.

Although the capacitor element 904 is illustrated in FIGS. 9A to 9D andFIG. 10, it is not necessary that the capacitor element 904 be providedwhen the gate capacitance of the transistor or another parasiticcapacitance is enough.

As described above, various types of equivalent circuits of a pixel of alight emitting device according to the invention can be employed.

Embodiment Mode 11

In this embodiment mode, a passive type light emitting device isexplained, which is different from that in the above embodiment mode.

As shown in FIG. 19, a base insulating film 311 is provided over asubstrate 600, and a first conductor 312 and a second conductor 313 tobe an electrode are stacked. Current is supplied to a light emittingelement 603 by controlling the electrode, and accordingly, display canbe conducted. The light emitting element 603 is arranged in matrix andtwo-dimensionally, which is included in a screen which displays animage.

A signal which controls an electrode is formed by an IC chip 323 whichis connected through anisotropic conductive materials 324 and 325. Inaddition, an external signal or the like is input to the IC chip 323through a flexible printed circuit 716 which is connected by ananisotropic conductive material 715.

The sealing of the light emitting element 603 is conducted by apassivation film 713, a sealing medium 317, and an opposing substrate610. The passivation film 713 is formed of an insulating film which isdifficult to penetrate water vapor, such as a silicon nitride film. Thelight transmittance of the silicon nitride film is slightly lowered in anear-ultraviolet region; therefore, a silicon nitride oxide film addedwith oxygen may be used to improve the light transmittance. In addition,aluminum nitride or aluminum nitride oxide may be applied to thepassivation film 713. The opposing substrate 610 may be formed frommetal such as stainless steel or aluminum, besides glass, plastic, orthe like. In the case where light of the light emitting element 603 isemitted from an opposing substrate 610 side, glass or plastic whichtransmits light is preferably used for the opposing substrate 610.Acrylic, polyethylene terephthalate (PET), or the like can be used forplastic, and a plate-like or film-like plastic can be used. In the casewhere plastic is used for the opposing substrate 610, a gas barrier filmwhich shields water vapor or the like or a hard coat film whichincreases the hardness of the surface may be provided. The sealingmedium 317 provided between the opposing substrate 610 and thepassivation film 713 is formed from a resin material such as an epoxyresin, a silicone resin, a phenol resin, or an urethane resin. Thesealing medium 317 fixes the opposing substrate 610 and the substrate600 and keeps a fixed distance from the opposing substrate 610 to thesubstrate 600. For that purpose, a silica particle or the like which isto be a spacer may be included in the sealing medium 317. According tothis structure, intrusion of moisture or the like which causesdeterioration of the light emitting element 603 can be prevented.

Further, the light emitting element 603 is a stacked layer type lightemitting element; therefore, luminous efficiency can be enhanced.Besides, a distance between a light emitting layer and a reflectiveelectrode in each light emitting element is approximately oddlymultiplied ¼ wavelength to enhance luminous output efficiency.Therefore, the amount of current which is applied can be kept low andthe lifetime of the light emitting element can be improved.

The passive type light emitting device has a structure in which asemiconductor element is not provided at the intersecting portion of ascanning line and a signal line in a pixel portion; therefore, apertureratio can be raised.

In addition, by providing color filters for the opposing substrate 610or the like, full color display can be conducted.

Embodiment Mode 12

In this embodiment mode, a television receiver to which a light emittingdevice according to the invention is applied is explained.

FIG. 11 shows a module in which a light emitting device according to theinvention and a circuit board 802 are combined. The circuit board 802 isprovided with, for example, a control circuit, a signal dividingcircuit, and the like. The light emitting device is manufacturedaccording to the above embodiment mode.

The light emitting devices includes a pixel portion 720 in which a lightemitting element is provided in each pixel, first and second scanningline driver circuits 721 and 722, and a signal line driver circuit 723supplying a video signal to a selected pixel. Further, a signal is sentfrom the circuit board 802 to the light emitting device through aflexible printed circuit 716. The circuit board 802 is provided with acontrol circuit 814 and a signal dividing circuit 815.

A high precision television receiver with low power consumption can becompleted by mounting the light emitting device according to theinvention.

FIG. 12 is a block diagram showing a principal structure of thetelevision receiver. As shown in FIG. 12, a configuration of an externalcircuit formed in the circuit board 802 includes a video signalamplifier circuit 812 which amplifies a video signal among signalsreceived in a tuner 811; a video signal processing circuit 813 whichconverts the signal output from the video signal amplifier circuit 812into a color signal corresponding to each color of red, green, and blue;a control circuit 814 which converts the video signal into input signalof a driver IC; and the like on an input side of a video signal. Asignal is output from the control circuit 814 to the first and secondscanning line driver circuits 721 and 722 and the signal line drivercircuit 723, respectively. In the case of conducting digital driving, asignal dividing circuit 815 is provided between the signal line drivercircuit 723 and the control circuit 814 to have a structure in which aninput digital signal is divided into m pieces and supplied.

An audio signal among signals received by the tuner 811 is sent to anaudio signal amplifier circuit 816 and the audio signal is supplied to aspeaker 818 through an audio signal processing circuit 817. A controlcircuit 819 receives control information on sound volume and a receivingstation (receiving frequency) from an input portion 820, and a signal issent to the tuner 811 and the audio signal processing circuit 817.

As shown in FIG. 13, a television receiver can be completed byincorporating the light emitting device mounted with the externalcircuit into a chassis 831. A display screen 832 is formed by using thelight emitting device. In addition, as an accessory equipment, a speaker818, operation switches 834, and the like are appropriately provided.Thus, a television receiver can be completed by applying the invention.

The television receiver can display an image which is clear and superiorin image quality by including a light emitting device.

Embodiment Mode 13

An electronic device which is provided with a light emitting deviceaccording to the present invention in a display portion includes: atelevision receiver, a camera such as a digital camera or a digitalvideo camera, a mobile phone set (simply referred to as a cellular phoneset or a cellular phone), a portable information terminal such as a PDA,a portable game machine, a monitor for a computer, a computer, a soundreproducing device such as a car audio set, an image reproducing deviceprovided with a recording medium such as a home game machine, and thelike. Specific examples thereof will be described with reference toFIGS. 14A to 14E.

A portable information terminal device shown in FIG. 14A includes a mainbody 9201, a display portion 9202, and the like. The light emittingdevice according to the invention can be applied to the display portion9202. Accordingly, it is possible to provide a portable informationterminal device which can display an image which is clear and superiorin image quality and operates with low power consumption.

A digital video camera shown in FIG. 14B includes a display portion9701, a display portion 9702, and the like. The light emitting deviceaccording to the invention can be applied to the display portion 9701.Accordingly, it is possible to provide a digital video camera which candisplay an image which is clear and superior in image quality andoperates with low power consumption.

A cellular phone shown in FIG. 14C includes a main body 9101, a displayportion 9102, and the like. The light emitting device according to theinvention can be applied to the display portion 9102. Accordingly, it ispossible to provide a cellular phone which can display an image which isclear and superior in image quality and operates with low powerconsumption.

A portable television set shown in FIG. 14D includes a main body 9301, adisplay portion 9302, and the like. The light emitting device accordingto the invention can be applied to the display portion 9302.Accordingly, it is possible to provide a portable television set whichcan display an image which is clear and superior in image quality andoperates with low power consumption. Further, the light emitting deviceaccording to the invention can be applied to various types of portabletelevision sets such as a small-sized television incorporated in aportable terminal such as a cellular phone or a medium-sized televisionwhich is portable.

A portable computer shown in FIG. 14E includes a main body 9401, adisplay portion 9402, and the like. The light emitting device accordingto the invention can be applied to the display portion 9402.Accordingly, it is possible to provide a portable computer which candisplay an image which is clear and superior in image quality andoperates with low power consumption.

The electronic device can display an image which is clear and superiorin image quality and operates with low power consumption by including alight emitting device.

Embodiment 1

In this embodiment, an element structure in which a light emitting unitwhich exhibits blue, a light emitting unit which exhibits green, and alight emitting unit which exhibits red are sequentially stacked from afirst electrode 101 is explained.

An electrode having high reflectivity and comprising aluminum is usedfor the first electrode 101, and an electrode having a highlight-transmitting property and comprising indium tin oxide containingsilicon oxide is used for a second electrode 102.

In the light emitting unit which exhibits blue, a first layer 111B isformed from indium tin oxide containing silicon oxide; a second layer112B is formed of a layer in which an evaporated layer of α-NPD, anevaporated layer of t-BuDNA, and an evaporated layer of Alq₃ aresequentially stacked; and a third layer 113B is formed of aco-evaporated layer of BzOs and Li are used. In the light emitting unitwhich exhibits green, a first layer 111G is formed of a layer in whichα-NPD and a molybdenum oxide are mixed (also referred to as aco-evaporated layer since the layer is formed by a co-evaporationmethod); a second layer 112G is formed of a layer in which an evaporatedlayer of α-NPD, a co-evaporated layer of Alq₃ and coumarin 6, and anevaporated layer of Alq₃ are sequentially stacked; and a third layer113G is formed of a co-evaporated layer of BzOs and Li are used. Themass ratio of Alq₃:coumarin 6 is set so as to be 1:0.005.

In the light emitting unit which exhibits red, a first layer 111R isformed of a layer in which α-NPD and a molybdenum oxide are mixed usingα-NPD as an organic compound (also referred to as a co-evaporated layersince the layer is formed by a co-evaporation method); a second layer112R is formed of a layer in which an evaporated layer of α-NPD and aco-evaporated layer of Alq₃, rubrene, and DCJTI are sequentiallystacked; and a third layer 113R is formed of a co-evaporated layer ofBzOs and Li are used. The mass ratio of Alq₃:rubrene:DCJTI is set so asto be 1:1:0.02. Further, the mass ratio of BzOs:Li used for the thirdlayer 113 of each light emitting element is set so as to be 1:0.01.Furthermore, the mass ratio of a molybdenum oxide:α-NPD used for thefirst layer 111 is set so as to be 1:0.25.

As described above, indium tin oxide containing silicon oxide can beapplied to the first layer 111B for controlling a distance from a lightemitting layer to the first electrode 101. In that case, a layer formedfrom a material which is superior in a hole injecting property such asDNTPD may be provided between the first layer 111B formed from indiumtin oxide containing silicon oxide and the second layer 112B.

One feature of a light emitting element according to the embodiment isthat, in a light emitting unit which exhibits green and a light emittingunit which exhibits red, a layer in which an organic compound typifiedby α-NPD and a molybdenum oxide are mixed is used for a layer forcontrolling a distance from a light emitting layer to a first electrode101. It has been revealed that a drive voltage does not rise even if thelayer in which a molybdenum oxide and an organic compound are mixed isthickened. Therefore, the layer in which a molybdenum oxide and anorganic compound are mixed is preferably used in a light emittingelement with light emitting units each exhibit green and red are stackedin order to control the distance from each light emitting layer (a layerformed from t-BuDNA, a layer formed from Alq_(a) and coumarin 6, and alayer formed from Alq₃, rubrene, or DCJTI) to the first electrode 101formed from aluminum since the film thickness can be large withoutincreasing driving voltage.

Embodiment 2

An element structure is explained in this embodiment, in which a mixedlayer of α-NPD and a molybdenum oxide is used for a first layer 111B forcontrolling a distance from a light emitting layer to a first electrode101 instead of indium tin oxide containing silicon oxide and a lightemitting unit which exhibits blue, a light emitting unit which exhibitsgreen, and a light emitting unit which exhibits red are sequentiallystacked from a first electrode 101 using the same material as in theabove embodiment.

One feature of a light emitting element according to this embodiment isthat a mixed layer of an organic compound typified by α-NPD and amolybdenum oxide is used for a layer for controlling the distance fromthe light emitting layer to the first electrode 101. It has beenrevealed that drive voltage does not rise even if the mixed layer of amolybdenum oxide and an organic compound is thickened. Therefore, thelayer in which a molybdenum oxide and an organic compound are mixed ispreferably used in all light emitting elements in which light emittingunits are stacked in order to control the distance from each lightemitting layer (a layer comprising t-BuDNA, a layer comprising Alq₃ andcoumarin 6, and a layer comprising Alq₃, rubrene, and DCJTI) to thefirst electrode 101 formed from aluminum since of a film thickness canbe large.

Embodiment 3

In this embodiment, luminance is compared using a stacked layer typelight emitting element and a single-layer type light emitting elementaccording to the present invention by simulation. The stacked layer typelight emitting element has a structure in which light emitting units 100a and 100 b which exhibit green are stacked between a first electrode101 and a second electrode 102 as shown in FIG. 18A. Each of the lightemitting elements 100 a and 100 b which exhibit green has first layers111 a and 111 b, second layers 112 a and 112 b, and third layers 113 aand 113 b, the first to third layers are formed from the same materialsas in the above Embodiment 1, the first layer 111 a is formed of a layerhaving indium tin oxide containing silicon oxide, and the first layer111 b is formed of a mixed layer of α-NPD and a molybdenum oxide. Thefirst electrode 101 is formed from aluminum and the second electrode 102is formed from indium tin oxide containing silicon oxide. Simulation isconducted under the following condition: the first electrode 101 isformed in 100 nm thick; the first layer. 111 a formed from indium tinoxide containing silicon oxide, in 40 nm thick; an evaporated layer ofα-NPD, in 10 nm thick; a co-evaporated layer of Alq₃ and coumarin 6, in40 nm thick; an evaporated layer of Alq₃, in 20 nm thick, to form secondlayers 112 a and 112 b each formed of the three layers; a co-evaporatedlayer of BzOs and Li, in 20 nm thick, to form third layers 113 a and 113b; the first layer 111 b in which α-NPD and a molybdenum oxide aremixed, in 30 nm thick; and the second electrode 102, in 110 nm thick.

The single-layer type light emitting element has a structure including alight emitting unit 100 a which exhibits green between a first electrode101 and a second electrode 102 as shown in FIG. 18B. The light emittingunit 100 a which exhibits green has a first layer 111 a, a second layer112 a, and a third layer 113 a, the first to third layers are formedfrom the same materials as these of the elements shown in FIG. 18A, andthe first layer 111 a is formed of a layer having indium tin oxidecontaining silicon oxide.

FIG. 16 is a graph of luminance with respect to a wavelength (nm) in thelight emitting element shown in FIGS. 18A and 18B. An element A is asingle-layer type light emitting element which is the light emittingelement shown in FIG. 18B, in which the distance from the firstelectrode 101 to the light emitting layer is not approximately oddlymultiplied ¼ wavelength by controlling a thickness of the first layer111. An element B is a top side light emitting element of a stackedlayer type light emitting element which is the light emitting elementshown in FIG. 18A, in which the distance from the first electrode 101 tothe light emitting layer is approximately oddly multiplied ¼ wavelengthby controlling a thickness of the first layer 111. An element C is abottom side light emitting element of a stacked layer type lightemitting element which is the light emitting element shown in FIG. 18A,in which the distance from the first electrode 101 to the light emittinglayer is approximately oddly multiplied ¼ wavelength by controlling athickness of the first layer 111. An element D means a result ofcombining the luminance of the element B and the element C.

By comparing the element A with the element B or element C in FIG. 16,it is revealed that luminance is increased in the case where thedistances from the first electrode 101 to the light emitting layers areapproximately oddly multiplied ¼ wavelength by controlling a thicknessof the first layer 111 a having indium tin oxide containing siliconoxide and by the first layer 111B in which α-NPD and a molybdenum oxideare mixed in the stacked layer type light emitting element.

FIG. 17 is a graph of luminance with respect to a wavelength (nm), whichis a result of conducting simulation using a mixed layer of α-NPD and amolybdenum oxide as the first layer 111 a shown in FIGS. 18A and 18B. Inother words, the conditions of the elements A to D are different from acase of FIG. 16 in terms of having the first layer 111 a formed of amixed layer of α-NPD and a molybdenum oxide. The first layer 111 aformed of a mixed layer of α-NPD and a molybdenum oxide in the element Ahas a film thickness of 35 nm.

By comparing the element A with the element B or element C in FIG. 17,it is revealed that luminance is increased in the case where thedistance from the first electrode 101 to the light emitting layer isapproximately oddly multiplied ¼ wavelength by controlling a thicknessof the first layers 111 a and 111 b in which α-NPD and a molybdenumoxide are mixed.

Further, by comparing FIG. 17 with FIG. 16, it is revealed that thelight emitting element using the first layer 111 a formed of a mixedlayer of α-NPD and a molybdenum oxide has higher emission luminance.Furthermore, it is revealed that the first layer 111 a in which α-NPDand a molybdenum oxide are mixed is preferable since the first layer 111a has higher conductivity than that of a layer having only α-NPD anddrive voltage does not rise even if a film is thickened.

This application is based on Japanese Patent Application serial No.2005-013688 field in Japan Patent Office on Jan. 21, 2005, the contentsof which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting device comprising: a firstsubstrate; an anode over the first substrate; a first mixed layer overand in contact with the anode, the first mixed layer comprising anorganic compound and a metal oxide which exhibits an electron acceptingproperty to the organic compound; a first light-emitting layer over thefirst mixed layer; a second mixed layer over the first light-emittinglayer, the second mixed layer comprising the organic compound and themetal oxide; a second light-emitting layer over the second mixed layer;a layer over the second light-emitting layer, the layer comprising anelectron transporting material and a material which exhibits an electrondonating property to the electron transporting material; a cathode overand in contact with the layer; a color filter over the cathode; and asecond substrate over the color filter, wherein the first light-emittinglayer and the second light-emitting layer are different in emissioncolor from each other, wherein the anode is a reflective electrode. 2.The light-emitting device according to claim 1, wherein the anodecomprises aluminum.
 3. The light-emitting device according to claim 1,wherein the cathode comprises indium tin oxide.
 4. The light-emittingdevice according to claim 1, wherein the metal oxide is molybdenumoxide.
 5. The light-emitting device according to claim 1, wherein thefirst light-emitting layer comprises a first organic compound, andwherein the second light-emitting layer comprises a second organiccompound which is different from the first organic compound.
 6. Thelight-emitting device according to claim 1, wherein the color filter isin contact with the second substrate.
 7. The light-emitting deviceaccording to claim 1, wherein the second substrate is selected from aglass substrate, a plastic substrate, and a metal substrate.
 8. Thelight-emitting device according to claim 1, wherein a thickness of thefirst mixed layer is arranged so that an optical distance from the firstlight-emitting layer to the anode is odd multiples of quarter wavelengthof emission color of the first light-emitting layer with a range of plusor minus 10% thereof.
 9. The light-emitting device according to claim 1,wherein the light-emitting device emits white light.
 10. An electronicdevice comprising the light-emitting device according to claim 1,wherein the electronic device is selected from a television receiver, acamera, a mobile phone set, a portable information terminal, a portablegame machine, a monitor for a computer, a computer, a sound reproducingdevice, and an image reproducing device.
 11. A light-emitting devicecomprising: a first substrate; a first light-emitting element, a secondlight-emitting element, and a third light-emitting element over thefirst substrate, each of the first light-emitting element, the secondlight-emitting element, and the third light-emitting element comprising:an anode over the first substrate; a first mixed layer over and incontact with the anode, the first mixed layer comprising an organiccompound and a metal oxide which exhibits an electron accepting propertyto the organic compound; a first light-emitting layer over the firstmixed layer; a second mixed layer over the first light-emitting layer,the second mixed layer comprising the organic compound and the metaloxide; a second light-emitting layer over the second mixed layer; alayer over the second light-emitting layer, the layer comprising anelectron transporting material and a material which exhibits an electrondonating property to the electron transporting material; and a cathodeover and in contact with the layer; a red color filter over the firstlight-emitting element; a green color filter over the secondlight-emitting element; a blue color filter over the thirdlight-emitting element; and a second substrate over the red colorfilter, the green color filter, and the blue color filter, wherein thefirst light-emitting layer and the second light-emitting layer aredifferent in emission color from each other, wherein the anode is areflective electrode.
 12. The light-emitting device according to claim11, wherein the anode comprises aluminum.
 13. The light-emitting deviceaccording to claim 11, wherein the cathode comprises indium tin oxide.14. The light-emitting device according to claim 11, wherein the metaloxide is molybdenum oxide.
 15. The light-emitting device according toclaim 11, wherein the first light-emitting layer comprises a firstorganic compound, and wherein the second light-emitting layer comprisesa second organic compound which is different from the first organiccompound.
 16. The light-emitting device according to claim 11, whereinthe red color filter, the green color filter, and the blue color filtereach are in contact with the second substrate.
 17. The light-emittingdevice according to claim 11, wherein the second substrate is selectedfrom a glass substrate, a plastic substrate, and a metal substrate. 18.The light-emitting device according to claim 11, wherein a thickness ofthe first mixed layer is arranged so that an optical distance from thefirst light-emitting layer to the anode is odd multiples of quarterwavelength of emission color emitted from the first light-emitting layerwith a range of plus or minus 10% thereof.
 19. The light-emitting deviceaccording to claim 11, wherein each of the first light-emitting element,the second light-emitting element, and the third light-emitting elementemits white light.
 20. An electronic device comprising thelight-emitting device according to claim 11, wherein the electronicdevice is selected from a television receiver, a camera, a mobile phoneset, a portable information terminal, a portable game machine, a monitorfor a computer, a computer, a sound reproducing device, and an imagereproducing device.
 21. The light-emitting device according to claim 1,wherein a thickness of the first mixed layer is different from athickness of the second mixed layer.
 22. The light-emitting deviceaccording to claim 11, wherein a thickness of the first mixed layer isdifferent from a thickness of the second mixed layer.
 23. Alight-emitting device comprising: an anode; a first mixed layer over andin contact with the anode, the first mixed layer comprising an organiccompound and a metal oxide which exhibits an electron accepting propertyto the organic compound; a first light-emitting layer over the firstmixed layer, the first light-emitting layer comprising a firstlight-emitting material; a second mixed layer over the firstlight-emitting layer, the second mixed layer comprising the organiccompound and the metal oxide; a second light-emitting layer comprisingover the second mixed layer, the second light-emitting layer comprisinga second light-emitting material; a layer over the second light-emittinglayer, the layer comprising an electron transporting material and amaterial which exhibits an electron donating property to the electrontransporting material; and a cathode over and in contact with the layer,wherein the anode is a reflective electrode.
 24. The light-emittingdevice according to claim 23, wherein the anode comprises aluminum. 25.The light-emitting device according to claim 23, wherein the cathodecomprises indium tin oxide.
 26. The light-emitting device according toclaim 23, wherein the metal oxide is molybdenum oxide.
 27. Thelight-emitting device according to claim 23, wherein the firstlight-emitting layer comprises a first organic compound, and wherein thesecond light-emitting layer comprises a second organic compound which isdifferent from the first organic compound.
 28. The light-emitting deviceaccording to claim 23, wherein a thickness of the first mixed layer isarranged so that an optical distance from the first light-emitting layerto the anode is odd multiples of quarter wavelength of emission color ofthe first light-emitting layer with a range of plus or minus 10%thereof.
 29. The light-emitting device according to claim 23, whereinthe light-emitting device emits white light.
 30. An electronic devicecomprising the light-emitting device according to claim 23, wherein theelectronic device is selected from a television receiver, a camera, amobile phone set, a portable information terminal, a portable gamemachine, a monitor for a computer, a computer, a sound reproducingdevice, and an image reproducing device.